1. Field of the Invention
The present invention relates to a device for detecting the drop in air pressure of any of tires mounted on a vehicle.
2. Description of the Related Art
In recent years, as an example of a safety device of a four-wheel vehicle such as an automobile or a truck, devices for detecting the drop in air pressure of any of tires have been developed, and some of them have been put to practical use.
An example of a method of detecting the drop in air pressure of any of tires is a method utilizing a difference among the respective rotational angular velocities F1, F2, F3 and F4 of four tires W1, W2, W3 and W4 mounted on a vehicle. The tires W1 and W2 are respectively right and left front tires, and the tires W3 and W4 are respectively right and left rear tires.
In this detecting method, on the basis of a signal outputted from a wheel speed sensor mounted on each of the tires Wi (i=1, 2, 3 or 4), the rotational angular velocity Fi of the tire Wi is detected for each predetermined sampling period.
If the effective rolling radii of all the tires Wi are equal, and the vehicle is linearly traveling, the detected rotational angular velocities Fi of the tires Wi should be equal. The effective rolling radius is a value obtained by dividing the distance the tire Wi freely rolling in a loaded state advances by one rotation by 2.pi..
The effective rolling radius of the tire Wi changes depending on the change in the air pressure of the tire Wi, for example. That is, when the air pressure of the tire Wi drops, the effective rolling radius is smaller than that at the time of normal internal pressure. Consequently, the rotational angular velocity Fi of the tire Wi whose air pressure has dropped is higher than that at the time of normal internal pressure. Therefore, the drop in the air pressure of the tire Wi can be judged on the basis of the difference among the rotational angular velocities Fi.
A judgment expression used in detecting the drop in the air pressure of the tire Wi on the basis of the difference among the rotational angular velocities Fi is the following equation (1), for example (see JP-A-63-305011, JP-A-4-212609, etc.): ##EQU1##
If the effective rolling radii of all the tires Wi are equal, all the rotational angular velocities Fi are equal (F1=F2=F3=F4). Consequently, a judged value D is zero. Therefore, thresholds Dth1 and Dth2 (where Dth1 and Dth2&gt;0) are set. When the conditions given by the following expression (2) are satisfied, it is judged that the air pressure of any of the tires Wi has dropped. When the conditions are not satisfied, it is judged that all the tires Wi have normal internal pressure. EQU D&lt;-Dth1 or D&gt;Dth2 (2)
The effective rolling radii of the actual tires include a variation within a production tolerance of the tires (hereinafter referred to as an "initial difference"). That is, even if all the four tires Wi have normal internal pressure, the effective rolling radii of the four tires Wi differ depending on the initial difference. Correspondingly, the rotational angular velocities Fi of the tires Wi vary. As a result, the judged value D may be a value other than zero. Therefore, it may be erroneously detected that the air pressure has dropped, although it has not drop. In order to detect the drop in the air pressure with high precision, therefore, it is necessary to eliminate the effect of the initial difference from the detected rotational angular velocities Fi.
As a technique for eliminating the effect of the initial difference from the rotational angular velocities Fi, it is considered that a technique disclosed in JP-A-7-156621 is applied. In the technique disclosed in the unexamined publication, initial correction factors K1 to K3 for correcting a variation depending on an initial difference among tires Wi are found (this processing for finding the initial correction factors K1 to K3 is referred to as "STD (Straight Running Determination) Processing"). The rotational angular velocities Fi are corrected on the basis of the found initial correction factors K1 to K3. Consequently, the effect of the initial difference is eliminated from the rotational angular velocities Fi. It is Judged whether or not the air pressure of any of the tires has dropped on the basis of rotational angular velocities after the correction.
More specifically, in the above-mentioned proposed technique, the initial correction factors K1 to K3 are respectively found as the average values of the ratios of the rotational angular velocities of the right and left front tires, the right and left rear tires and the front and rear tires which are found in each sampling period, as expressed by the following equations (3) to (5), with the pressing of an STD switch by a driver taken as calculation starting conditions. In the following equations (3) to (5), n corresponds to the number of times of calculation. .SIGMA. denotes the total in the case of p=1 to p=n. EQU K1={.SIGMA.(F1/F2).sub.P }/n (3) EQU K2={.SIGMA.(F3/F4).sub.P }/n (4) EQU K3={.SIGMA.(F1/F3).sub.P }/n (5)
In order to thus find the initial correction factors K1 to K3, a certain time period is required. On the other hand, assuming that air pressure drop judgment processing is not performed until the final initial correction factors K1 to K3 are found, even if the air pressure of any of the tires drops during the STD processing, it is impossible to detect the drop in the air pressure.